The invention relates to a lighting system with a light source provided with a light bulb and with a reflector associated with the light source, in which an interference filter having a plurality of layers is provided on at least a portion of the outside surface of the light bulb and has a high reflectivity for infrared radiation.
It is known to use halogen incandescent lamps in different optical systems, such as, for example, studio lighting and operation-room lighting, or daylight film projectors, in which as little as possible of the infrared radiation content of the light is to issue in the direction of projection, in order to reduce thermal radiation in the illuminated field. This is accomplished as a rule by reflectors which transmit infrared radiation and reflect visible light and/or by filters at the openings at which the light emerges, which absorb much of the infrared radiation content.
DE-OS 32 27 096 discloses a cylindrical halogen incandescent lamp with an incandescent coiled filament held along the axis of the cylinder of the light bulb, in which the light bulb is coated on its exterior with a multi-layer body acting as an interference filter which reflects the infrared radiation content of the light produced in the lamp onto the coil, while it transmits the visible content of the produced light. The interference filter has layers of alternately high and low index of refraction, the materials of these layers consisting substantially of silicon dioxide and tantalum pentoxide. If the filament is sufficiently well adjusted along the axis of symmetry of the lamp bulb, the reflected infrared radiation is partially absorbed by the filament. In this manner the amount of transmitted infrared radiation is reduced, thereby improving the efficiency of the lamp.
A similar arrangement is disclosed in U.S. Pat. No. 4,689,519, according to which an incandescent lamp is provided in the cylindrical central portion of its elongated lamp bulb with an interference filter which reflects the infrared radiation produced by the filament to reduce heat losses in the lamp bulb; the filter consists of alternately disposed layers of low and high refractive index, these layers consisting of silicon dioxide and tantalum pentoxide; the two bulb ends are not coated with tantalum pentoxide, because reflection of infrared light onto the filament from these ends is ineffective.
Furthermore, DE-OS 15 89 095 discloses gas discharge lamps whose bulbs are provided with an optical interference filter of several partial layers of different refractive index for the purpose of producing a heat-resistant filter system for the production of neutral-color light.
Another application of interference filters is disclosed in DE-GM 18 09 322, in which a cold-light mirror transparent to heat rays is described, whose surface is covered with a series of interfering dielectric layers of alternately high and low refraction; silicon oxide and titanium oxide or tantalum oxide are used as the materials of the layers.
In the case of halogen incandescent lamps of relatively small dimensions it is not possible to absorb the infrared radiation in the filament by reflecting the infrared content onto an interference filter applied to the bulb, since the axis of symmetry of the filament does not, as a rule, coincide with the axis of symmetry of the lamp bulb, and in common forms of construction (e.g., bulbs with a base on one end) it is perpendicular thereto.
If nevertheless the bulb were to be coated with an interference filter it would result in multiple reflection which, however, due to the partial transmittance of the interference coating, would ultimately result in the transmission of the infrared radiation. Reduction of the infrared content would be practically unachievable.
The invention proposes to achieve a very simple shielding of the illuminated field against infrared radiation by geometrical coordination of the light source and the reflector, as well as by partially coating the outer surface of the bulb of the light source, wherein lamp and reflector are relatively inexpensive in spite of their excellent effectiveness.
One preferred embodiment is a halogen incandescent lamp with a base at one end, in which the lamp is so coated that infrared radiation issues from the base area or from the area of the dome of the light bulb or in both areas.
An advantage proves to be the elimination of the filter and filter holder, so that a relatively simple construction is the result, requiring no skilled maintenance. This is advantageous in the case of operating-room lighting with a plurality of individual light sources. Furthermore, the stocking of replacements is simplified, since separate infrared radiation absorbing filters are no longer necessary.